What's causing these checker board-like machining marks ?

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compact8

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It's a hobby-grade EMCO FB-2 milling machine fitted with a 40mm diameter face milling head, 4 blades. Material is 7075 Aluminium alloy, cutting depth 0.1 mm ( finishing ) RPM 680, feeding speed 65 mm / 2.56 inch per min. All the inserts are very new. When I used them for the first time, there was no such checker board marks. What could have changed to cause this ? That said, these marks are no issue at all and I could not feel it with my fingers but I would like to know whether they are indicating the onset of any problems.

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First off it looks like your tram is perfect. The pattern is typical of what you should see, although it does look rough in the pic. You could try a higher spindle speed and slower feed. An insert with a slightly bigger nose radius would also help.
 
If those are carbide inserts that may be the problem since hobby machines often cannot run them at the speeds/feeds they are designed for.
Post a close up picture of one insert if you can.
I suspect they may not have enough positive rake
This issue comes up frequently with lathe users also
-Mark
 
markba633csi said:
If those are carbide inserts that may be the problem since hobby machines often cannot run them at the speeds/feeds they are designed for.
Post a close up picture of one insert if you can.
I suspect they may not have enough positive rake
This issue comes up frequently with lathe users also
-Mark
Click to expand...
Yes, these are carbide inserts designed for cutting Aluminium according to the vendor. The rake angle is rather large

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There are some other possible causes: did you have the head locked tightly during cutting? What about the machine's table?
Are the gibs adjusted snugly?
What are the condition of the headstock bearings? Is this a single phase motor or three phase (both were available)
All these things play a part, the FB-2 is a precise machine but not especially rigid due to it's structure
In addition, the head is gear drive which can introduce harmonic forces when cutting
And lastly, the inserts may have slight differences in height
-Mark
 
Are you using a cutting oil or kerosene or tapping fluid, WD 40? Anything is better than nothing.
 
You've got a harmonic at work there, Close examination shows that over a period of about two dozen cuts (a dozen digging in, then a dozen lifting out) your piece (or your cutter) vibrates up and down, leaving slight ridges and valleys. And the work also shows that either your cutter's teeth are set absolutely perfectly and your feed it too fast, or you have one tooth that is doing all the cutting and your feed is too slow for it to eliminate the harmony.

Make sure all table and head locks are snug, but not tight enough to bind and that your tool holder is tight in the mill, and tight on the arbor.

Your tram, as noted, is perfect, we'd all aspire to that degree of squareness.
 
The photos of your insert look like a steel cutting style.. I inserts for aluminum should be polished to a mirror finish. The recommendations for a higher RPM and slower feed rte are good ones. Ditto for WD40, kerosene or a mist of coolant.
 
I would double the RPMs( you're running way too slow for carbide/aluminum. ) and give it a toot of WD40 and see what happens. Looks like your head tram is dead on.
 
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